The subject matter of this invention relates generally to VAR generators and more specifically to static VAR generators employing switched inductors used in conjunction with switched capacitors where losses are minimized.
It is known to make VAR generators by connecting a fixed capacitor and a switched inductor in parallel across two lines of a voltage system to be regulated or controlled by the VAR generator. A suitable control system is provided for sending an output signal to the switch portion of the switched inductor to establish a conduction interval during a predetermined period of time. The conduction interval allows current to flow for a portion of the predetermined period of time, thus generating an inductively reactive current which interacts with fixed capacitively reactive current to produce a net reactive current which cooperates with the voltage across the lines to produce reactive power. The predetermined interval of time is usually one-half cycle of the line voltage. Consequently, in a half-cycle-by half cycle basis, the switching interval can be changed to provide differing amounts of reactive power as is determined to be necessary by the calculating or control portion of the system. Systems of the previous type can be found in U.S. Pat. No. 3,936,727, issued Feb. 3, 1976 to F. W. Kelly, Jr. and G. R. E. Laison and U.S. Pat. No. 3,999,117, issued Dec. 21, 1976 to L. Gyugyi et al. The latter patent is assigned to the assignee of the present invention. The values for capacitance and inductance are usually chosen in the prior art so that at a moderate conduction interval for the switched inductor, the thusly produced inductive current is approximately equal to the fixed capacitive current, thus producing zero VAR. Consequently, if the conduction interval is increased, the amount of inductive current increases, producing a net inductive reactive current. On the other hand, if the conduction interval is decreased, the inductive current is decreased, producing a net capacitive reactive current. This gives positive and negative VAR capability to the system. A system of this type has a number of problems, however. On problem lies in the fact that even at standby or a disposition of no VAR generation, appreciable power generation is required in each of the inductive and capacitive components of the system. Said another way, in a system of the type previously mentioned, significant inductive current is generated at the time when no VAR correction of production is required because the significant inductive current is utilized to cancel the oppositely phased capacitive current. This means that there are relatively high standby losses. Furthermore, for any given amount of VAR correction, either negative or positive, a minimum capacitance and inductance is required. An improvement on the aforementioned system includes utilization of an inductive branch and a capacitive branch in which the inductive branch operates independently of the capacitive branch, and vice versa. In this system, at standby, neither the inductive portion of the system nor the capacitive portion of the system conducts appreciable current and therefore the standby losses are lower than in the aforementioned system. Net inductive current is provided by using the inductive portion of the system exclusively; and net capacitive current is provided by using the capacitive portion of the system exclusively. However, the problem is present with this kind of system in that the capacitive branch of such a system is not conducive to continuous switch control over a wide range of capacitive currents, as is the case with the inductive portion of the system. In the prior art, therefore, the capacitive portion of such a system utilizes a bank of discrete capacitors, each having a separate switch. The net capacitive reactance for capacitive VAR production is provided by judiciously picking combinations of capacitors in the bank of capacitors to provide discrete values of capacitance. Nevertheless, such a system has the inherent disadvantage of only allowing discrete values of capacitive current to be produced. Thus, continuous control is difficult, if not impossible. In the range of capacitive VAR demand, only a relatively few values of capacitive current are available because of the discrete nature of the system. As a consequence, VAR compensation or correction in the capacitive current range tends to be an approximation. This problem was resolved in the prior art by providing a VAR generator with a continuous range of VAR correction which covers both negative and positive VAR generation. Inductive apparatus and capacitive apparatus are utilized in conjunction in such a manner that the inductive apparatus provides essentially the entire VAR generation for net inductive VAR demand but where discrete capacitors are utilized in conjunction with the same inductor to provide VAR generation over a continuous range for net capacitive demand. An appropriate control system decides the direction and magnitude of VAR demand. It has been found, however, that for certain types of systems to be compensated by the VAR generator it would be economically advantageous to keep the VAR output essentially zero to reduce losses when the terminal voltage variation is relatively small and only minimal compensation is required. It would be advantageous therefore if a VAR generator with attendant control system could be found which had an inactive VAR generating band around zero VAR's.